Without people, a yearly fire season in the Amazon Rainforest would be almost as unlikely as a hurricane season in Antarctica. Today, however, the Amazon is a frontier, and settlers are burning hundreds of thousands of acres of rainforest each year for cattle pasture and farms. Accidental fires degrade thousands of acres more.

The decisions about how the Amazon will be developed will mostly be made by Brazil, where 62 percent of the remaining forest is located. But on an increasingly crowded and climate-conscious planet, it’s inevitable that a human-made fire season in the world’s largest remaining rainforest—a treasure trove of species diversity and, if burned, a large source carbon dioxide emissions—will attract global attention.

Amazon attraction

Atmospheric scientist Ilan Koren of the Weizmann Institute of Science is among those whose attention gravitates toward the Amazon. The interest is part professional, part personal. After finishing high school and completing the three years of compulsory service in the army required by all Israeli young people, Koren says, “The last thing one wants to do is study.” Instead of heading straight to college, Koren set off with a friend to travel around the world. After mountain climbing in the Andes, they decided to end their tour with a motorcycle trip through the Amazon.

Bumping their way along dirt roads through the jungle and sleeping in tents, it took them about two months to get from Caracas, Venezuela, to Belem, Brazil, at the mouth of the Amazon River. “It seems like there are two reactions to the Amazon,” Koren says. “Either you cannot stand it because it is so humid and hot, with all these insects and strange animals, or else you fall in love with it. And I really fell in love with it—the humidity, the hot weather, everything.”

A natural cloud laboratory

The Amazon is also a fascinating natural laboratory for Koren’s primary research interest: clouds. “The clouds in the Amazon are special. The forest emits huge amounts of humidity,” Koren explains, “which creates a surrealistic view of mist and clouds forming from tree level all the way to the upper atmosphere—more than 10 kilometers up.” When the air over the Amazon is clean, the clouds that form over the forest are so like maritime clouds that scientists have referred to the Amazon as “the Green Ocean.”

Atmospheric scientist Ilan Koren is as at home collecting data on top of a 4,300-meter (14,000-foot) Colorado mountain as he is in front of his computer modeling the interactions between smoke and clouds. His love of the Amazon developed during a 3-month motorcycle trip he and a friend took through the area when he was in his 20s. (Photograph courtesy Lorraine Remer, NASA GSFC.)

The hundreds of thousands of fires that blaze during the dry season complicate that picture. For several years now, Koren has been collaborating with a team of scientists at NASA’s Goddard Space Flight Center, using use satellite data from a sensor called MODIS (short for Moderate Resolution Imaging Spectroradiometer) to better understand how smoke changes clouds in the Amazon. Smoke changes the size and number of cloud droplets, which affects their brightness. It also warms the smoke layer, which stabilizes the atmosphere and suppresses updrafts that fuel cloud formation. Clouds are better at reflecting sunlight back into space than smoke, so fires in the Amazon likely allow more solar energy than normal to enter the Earth’s climate system.

The unbroken canopy of the Amazon Rainforest releases so much water vapor into the air that the clouds over the forest are similar to those that form over the ocean. This satellite image from June 11, 2002, shows a uniform blanket of clouds interrupted only by the Amazon River and several of its tributaries. (Satellite image courtesy MODIS Rapid Response Team.)

When he first started studying these questions, Koren was focused on how smoke influenced clouds on a day to day basis. So, initially, he didn’t pay much attention to whether the amount of burning was changing over time. That began to change in 2005.

A bad year or a long-term trend?

“We had been working with the MODIS aerosol and cloud data since 2000, and we had noticed that there are large fluctuations [in smoke] between the years. Some years are relatively clean, and the next it is polluted. But in 2005, we began to be quite worried. We saw that it was so much more polluted, and we thought that we had to do something to try to understand what was happening,” Koren explained. “We wanted to know if that was just one bad year, or if fires and smoke in the Amazon were increasing.”

During the burning season, smoke often covers huge portions of the Amazon. Isolated towers of cumulus clouds poke through the dense layer of smoke in this photograph taken from an airplane. (Photograph courtesy Ilan Koren.)

“We decided to use two independent types of data. We used aerosol data from MODIS from 2000 to 2006 to measure smoke, and daily fire counts from NOAA’s AVHRR [Advanced Very High Resolution Radiometer] from 1998-2006.” says Koren. “We selected the entire biomass burning area, the heart of the Amazon where most of the fires occur, and we looked at daily and monthly averages in the number of fires and the amount of smoke. We compared the average of the dry season—August to November—of each year.”

“We also wanted to see it spatially,” he explains, “so we took each one degree of the data [an area of about 12,000 square kilometers] and mapped the trends over time.” What they saw surprised Koren. Since 1998, the number of fires in the dry season had nearly doubled. Smoke aerosols had increased by 60 percent. “We expected to see some increase, but such a strong increase was quite a shock.”

Since 1998, the number of fires occurring during the Amazon fire season nearly doubled. Smoke increased nearly 60 percent between 2000 to 2005. In 2006, the trend reversed, with both fires and smoke dropping off significantly. (Graph by Robert Simmon, based on data from Ilan Koren and Karla Longo.)

Equally surprising, though, was that in 2006, the upward trend suddenly reversed. The fire counts and smoke are among the lowest in the record. Why was 2006 so different? And what would 2007 bring?

After record burning in 2005, smoke levels dropped abruptly in 2006. Dark brown indicates high smoke aerosol concentrations, while light yellow indicates clearer skies. Smoke concentrations are higher in the southern Amazon, the forest’s agricultural frontier. (Maps by Robert Simmon, based on MODIS data from Ilan Koren.)